1. Field
Example embodiments relate to a nonvolatile memory device and method of fabricating the same, for example, a nonvolatile variable resistance memory device that may achieve more stable bipolar switching characteristics by forming two or more continuous layers using Cu2O and a Cu compound.
2. Description of the Related Art
In order to achieve a smaller size and a larger data storage capacity, semiconductor memory devices may be required to have a larger number of memory cells per unit area, a higher integrity, a faster operating speed, and a lower driving power.
Conventional semiconductor memory devices may include numerous memory cells which may be connected through circuit boards. Dynamic Random Access Memories (DRAM's) are representative volatile semiconductor memory devices. DRAM's may include a unit memory cell that may include a single switch and a single capacitor. DRAM's may have advantages of higher integrity and faster operating speed, but may have a disadvantage in that the DRAM may lose all stored data if the power is turned off.
Nonvolatile memory devices may preserve stored data even if the power is turned off. Flash memories are representative nonvolatile memory devices. Flash memories may have nonvolatile characteristics, unlike volatile memories, but may have disadvantages of lower integrity and slower operating speed.
Examples of nonvolatile memory devices include Magnetic Random Access Memories (MRAM's), Ferroelectric Random Access Memories (FRAM's), Phase-change Random Access Memories (PRAM's), and Resistance Random Access Memories (RRAM's).
Among these nonvolatile memory devices, RRAM's may use voltage-dependent resistance characteristics (variable resistance characteristics) of transition metal oxide (TMO). RRAM devices using TMO may have switching characteristics that may be suitable for memory devices.
FIG. 1A illustrates a conventional RRAM device using a variable resistance material. Referring to FIG. 1A, a conventional RRAM device may include a lower electrode 10, an oxide layer 12, and an upper electrode 14 that may be sequentially stacked on a substrate (not shown). The lower electrode 10 and the upper electrode 14 may be formed of a conductive material. The oxide layer 12 may be formed of transition metal oxide having variable resistance characteristics. For example, the oxide layer 12 may be formed of Cu2O, ZnO, TiO2, Nb2O5, ZrO2, CeO, VO, V2O5, NiOx, or the like.
With respect to perovskite-RRAM's, perovskite oxide may be used as a variable resistance material. For example, perovskite oxide that may be used in formation of an oxide layer may include PCMO (PrCaMnO3), Cr-STO (SrTiO3), etc. Perovskite-RRAM's may achieve memory characteristics according to polarity applied to a memory node using the principle of the Schottky barrier deformation.
FIG. 1B is a graph illustrating the voltage-current characteristics of the variable resistance memory device of FIG. 1A in which the oxide layer 12 is made of Cu2O. Referring to FIG. 1B, the variable resistance memory device using Cu2O may exhibit bipolar characteristics, but may have a larger current and voltage distribution and a higher current level at an “on” state.